1. Field of the Invention
The present invention relates to an audio circuit.
2. Description of the Related Art
In order to drive an electroacoustic conversion element such as headphones, speakers, or the like, an audio amplifier is employed. FIG. 1 is a circuit diagram showing an audio circuit investigated by the present inventor. An audio circuit 100r includes an audio amplifier 10. The audio amplifier 10 amplifies an analog audio signal (which will also be referred to as the “input voltage”) VIN so as to supply an output voltage VOUT to an electroacoustic conversion element 202 connected to an output terminal OUT.
The audio amplifier 10 shown in FIG. 1 is configured as an inverting amplifier including an operational amplifier 12 and resistors R11 and R12. The audio amplifier 10 amplifies the input voltage VIN. The input voltage VIN is configured as the sum total of the DC component VDC and the AC component (audio component) VAC. For simplicity of description and ease of understanding, description will be made assuming that VDC=0, and accordingly, that the audio signal VIN changes between a positive value and a negative value with 0 V as its center. The operational amplifier 12 is arranged such that a bias voltage VB=0 V is supplied to its non-inverting input terminal. Assuming that the operational amplifier 12 is an ideal operational amplifier having no input offset voltage VOFS the output voltage VOUT is represented by the following Expression (1).VOUT=−R12/R11×VIN  (1)
In actuality, the operational amplifier 12 has such an input offset voltage VOFS. In this case, the output voltage VOUT is represented by the following Expression (2).VOUT=−VOFS=R12/R11×(VIN+VOFS)  (2)
Accordingly, when the audio circuit 100r is started up, the output voltage VOUT changes toward a voltage represented by (−VOFS−R12/R11×VOFS) even if the input voltage VIN is zero, i.e., even in a silent state. This leads to a problem of noise (which will also be referred to as “pop noise”) output from the electroacoustic conversion element 202.
In order to suppress such pop noise, an approach is conceivable in which, in the manufacturing process, the offset voltage VOFS of the operational amplifier is adjusted by means of an adjustment method such as laser trimming or the like such that the offset voltage VOFS approaches zero. For example, the operational amplifier 12 is configured such that the bias current that flows through its differential input stage can be adjusted. Such an arrangement allows the offset voltage VOFS to be adjusted such that it becomes zero by means of laser trigging.
Typically, the trimming step is performed before the assembly step. However, in some cases, the offset voltage VOFS of the operational amplifier changes due to stress applied from an LSI package. Accordingly, in a case in which the offset voltage is adjusted such that it matches zero before the assembly step in a state in which the operational amplifier remains on a wafer, such an operational amplifier has the potential to have a non-zero offset voltage. Accordingly, with such an operational amplifier, pop noise can potentially occur.
Furthermore, the offset voltage VOFS depends on the power supply voltage supplied to the operational amplifier. Accordingly, in a case in which there is a difference between the power supply voltage supplied in the trimming step and the power supply voltage in actual use, such an arrangement has a problem of offset voltage in actual use. Accordingly, with such an operational amplifier, pop noise can potentially occur.